1) Field of the Disclosure
The disclosure relates generally to solar cells, and more specifically to inverted metamorphic solar cells and thin multijunction solar cells.
2) Description of Related Art
Solar cells or photovoltaic cells are devices designed to convert available light into electrical energy. Assemblies of such cells may be used to make solar panels, solar modules, or photovoltaic arrays, which can, in turn, be used to provide electrical power for a variety of applications such as satellites, spacecraft, and other space related applications; solar powered aircraft, high altitude unmanned aircraft, and other aircraft applications; and other suitable applications. In such applications, size, weight, and cost are factors to be considered, for example, the size, weight and cost of a satellite power system may be dependent on the energy and power conversion efficiency of the solar cells used.
A solar cell may include a flat photovoltaic wafer made from p-type or n-type crystalline semiconductor material, such as silicon, gallium arsenside, germanium, or another suitable material, in or on which a thin surface layer of the opposite conductivity type may be formed. The interface between the surface layer and the main region of the wafer defines a semiconductor junction. Solar cells may be modeled as diodes that respond to illumination by becoming forward biased and establishing a voltage across the cell. In addition, multijunction solar cells, formed from a combination of group III to group V materials and commonly referred to as Advanced “III-V” cells, may produce a somewhat higher forward voltage.
The solar cells may be assembled into arrays of solar cells connected together in series strings to provide a desired voltage, in parallel to provide a desired current, or in a series-parallel combination. When all of the solar cells in an array are illuminated, the solar cells are forward biased, and they each produce their respective voltage or current outputs which sum together to maintain the desired overall output. However, if one or more of the solar cells becomes shadowed, or not illuminated, those cells may become reverse biased due to the voltage generated by the unshadowed cells. For example, a spacecraft antenna may cast a shadow across an array. The effect of shadowing a solar cell in a series string depends upon the specific characteristics of the solar cell. If the solar cell has a very low reverse current, reverse biasing the cell can effectively force the string output to zero. If the cell electrically breaks down at a relatively low reverse voltage, the effect of shadowing a solar cell on the string output is reduced. Reverse biasing of a cell can cause permanent degradation or damage in cell performance or even complete cell failure.
Bypass diodes, typically made of silicon, may be used to minimize output losses and to protect cells against reverse bias when they become shadowed. Bypass diodes may be connected across single cells, across strings of cells, or across rows of parallel-connected cells. Bypass diodes that have very low reverse currents can avoid reducing current in the solar cell during normal operation, which can reduce power efficiency. When the cell becomes shadowed, the current flow through the cell may be limited, causing the cell to become reverse biased. This can cause the bypass diode to become forward biased. Most of the current can flow through the bypass diode rather than through the shadowed cell, thus allowing current to continue flowing through the array. The bypass diode can limit the reverse bias voltage across the cell thereby protecting the shadowed cell.
Known methods have been used to provide solar cells, including inverted metamorphic (IMM) solar cells, with bypass diode protection. One known method uses a bypass diode in the form of a discrete silicon flat diode. Such known bypass diode is typically separately mounted to a back surface of the solar cell. The bypass diode can protrude from the back surface of the solar cell thus making bonding more difficult. When the solar cell is cracked, portions of the solar cell may not be protected by the bypass diode. In addition, the bypass diode may be affixed to a corner of the solar cell thus making automated handling a challenge. Such known method may result in complex and inefficient assembly because the method may require the connection of adjoining cells to be formed by the assembler of the array rather than the cell manufacturer, thus resulting in increased cost.
Another known method uses a bypass diode in the form of a monolithic diode that can be built into the solar cell epitaxial (EPI) structure, wherein the epitaxial structure is defined for purposes of this application as the growth of a crystalline film on a crystalline substrate such that the film and substrate have the same structural orientation. Inverted metamorphic (IMM) solar cells or other thin multijunction cells may require a handle substrate for ease of handling. A handle substrate, or base substrate, of such known bypass diode is typically a separate component that is bonded to the solar cell or IMM solar cell structure. The handle substrate is used to provide mechanical support for the thin solar cell structure. Typically, the handle substrate may be made of germanium. However, germanium is a dense material and can be expensive, heavy, and brittle. Increased weight can affect mass sensitive missions by spacecraft or other high specific power applications, and the handle substrate may have decreased robustness due to the brittleness of germanium. Moreover, the known monolithic diode may be difficult to incorporate into the solar cell, IMM solar cell, or other cell structure and may be unstable in higher operating temperature applications.
In addition, methods for attaching the handle substrate to solar cell or thin IMM solar cell structures are known. Such known methods may include direct bonding or soldering. However, such known methods can result in electrical performance degradation, breakage, poor bond uniformity, unknown thermal cycle performance, and increased expense, and may use low throughput bonding equipment.
Accordingly, there is a need for a thin multijunction solar cell or inverted metamorphic (IMM) solar cell and method that provides advantages over known solar cells and methods.